Laboratory Simulations

Meierhenrich et al. (2001) and Muñoz Caro et al. (2002) produced amino acids by simulating interstellar conditions in the laboratory, using hard UV irradiation of a mixture of H2O, CO, CO2, CH3OH, and NH3 ices at a temperature T « 12 K. Upon warming the ice residue to room temperature, they identified 16 different amino acids, using gas chromatography-mass spectrometry (GC-MS). Some of these amino acids have also been identified in carbonaceous chondrites. In addition, they found pyrroles and furans in the residues. The products were confirmed by 13C-labelling of the ice. Chiral amino acids showed enantiomeric separation.

Muñoz Caro and Schutte (2003) performed a detailed quantitative infrared analysis of complex organic refractory material produced by photo- and thermal processing of interstellar ice analogs. They investigated the effects of the most relevant free parameters, such as ice composition, UV dose, photon energy, and temperature. They presented evidence for the presence of carboxylic

Table 4.1. Detected Interstellar Ice Absorption Features. (From Web page of Boogert, 2004).

A (|lm)

Identification

Object

Reference

2.27

CH 3 OH

W33A

Taban et al. (2003)

2.70

CO2

S140:IRS1

Keane et al. (2001a)

2.78

CO2

S140:IRS1

Keane et al. (2001a)

2.96

NH3, H2O

BN

Knacke et al. (1982)

3.07

H2O

BN

Gillett and Forrest (1973)

3.2-3.7

NH3, H2O

BN, +

Merrill et al. (1976)

3.25

PAH(?), NH4+(?)

Mon R2:IRS3

Sellgren et al. (1994)

3.47

-CH, NH4+(?)

W33A, +

Allamandola et al. (1992)

3.54

CH3OH

W33A

Grim et al. (1991)

3.85

CH3OH

W33A

Geballe, et al. (1985)

3.94

CH3OH

W33A

Geballe, et al. (1985)

4.1

CH3OH

W33A

Teixeira et al. (1999)

4.27

CO2

GL 2136, +

de Graauw et al. (1996)

4.38

13CO2

GL 2136

de Graauw et al. (1996)

4.5

H2O

Elias 29

Boogert et al. (2000)

4.62

XCN (=OCN" ?)

W33A

Lacy et al. (1984)

4.67

CO

W33A

Soifer et al. (1979)

4.78

13CO

NGC 7538:IRS9

Boogert et al. (2002)

4.92

OCS

W33A

Geballe, et al. (1985)

5.83 broad

HCOOH

NGC 7538:IRS9

Schutte et al. (1996)

5.83 narrow

H2CO

W33A, +

Keane et al. (2001b)

6.0

H2O, +

W51:IRS2

Puetter et al. (1979)

6.25

PAH(?)

NGC 7538:IRS9

Schutte et al. (1996)

Table 4.1. continued. (From webpage of Boogert, 2004).

A(|lm)

Identification

Object

Reference

6.85

CH3OH, NH4+(?), & others

W51:IRS2

Puetter et al. (1979)

7.25

HCOOH+ (?)

W33A

Schutte et al. (1997)

7.41

HCOO-(?), CH3HCO(?)

W33A

Schutte et al. (1997)

7.60

SO2 (?)

W33A

Boogert et al. (1997)

7.67

CH4

NGC 7538: IRS9

Lacy et al. (1991)

8.9

CH3OH

GL 2136

Skinner et al. (1992)

9.01

NH3

NGC 7538: IRS9

Lacy et al. (1998)

9.7

CH3OH

GL 2136

Skinner et al. (1992)

13.6

H2O

AFGL 961

Cox (1989)

15.2

CO2

GL 2136, +

de Graauw et al. (1996)

44.

H2O

RAFGL 7009S

Dartois et al. (1998)

acid salts and the formation of hexamethylenetetramine [HMT, (CH2)6N4] after warming to room temperature. The analysis of the products by GC-MS led to the detection of nitrogen-heterocyclic species, sulfur-bearing molecules, and amines. Figure 4.1 shows the gas chromatogram obtained from a mixture of photolyzed ices in the ratios H2O:CH3OH:NH3:CO:CO2=2:1:1:1. Figure 4.2 shows the determination of alinine enantiomers in the same sample. In another experiment, using a mixture of H2O:CO:NH3:H2S = 2:1:1:0.04, sulfur-polymerization was found to be efficient while other detected species, like pentathian (S5CH2), show chemical interaction between the C and the S elements. Some of these species are of prebiotic interest and may exist in comets.

Muñoz Caro et al. (2004) simulated experimentally the physical conditions present in dense interstellar clouds by means of a high vacuum experimental setup at a low temperature of T «12 K. The accretion and photoprocessing of ices on grain surfaces was simulated by depositing an ice layer with composition analogous to interstellar ices on a substrate window, while irradiating it with UV light. Upon slowly warming a sample to room temperature, a residue was obtained that contained the most refractory products of photo- and thermal processing. Fourier transform-infrared (FT-IR) spectroscopy and GC-MS were carried out on the refractory organic material that had formed under

Table 4.2. Comparison of Interstellar Ices to Cometary Ices (Modified from Alla-mandola et al., 1999).

Molecule Interstellar abundance Cometary abundance

H2O

100

100

CO (polar)

1-10

CO (nonpolar)

10-40

6

CH3OH

<4-20

1.7

CO2

1-10

5

XCN

1-10

NH3

5-10

0.9

H2

1

CH4

1

0.7

HCO

1

H2CO

1-4

1.8

N2

10-40

1

O2

10-40

OCS

few

0.2

a wide variety of initial conditions such as ice composition, UV spectrum, UV dose, and sample temperature. The refractory products obtained in these experiments were identified, and the corresponding efficiencies of formation were recorded. They found the first evidence for carboxylic acid salts as part of the refractory products. The features in the IR spectrum of the refractory material were attributed to hexamethylenetetramine [HMT, (CH2)6N4], ammonium salts of carboxylic acids [(R-COO")(NH+)], amides [H2NC(=O)-R], esters [R-C(=O)-O-R'] and species related to polyoxymethylene [POM, (-CH2O-)„]. Also a number of molecules based on HMT were identified: methyl-HMT (C6HnN4-CH3), hydroxy-HMT (C6HnN4-OH), methanyl-HMT (C6HnN4-CH2OH), amin-aldehyd-HMT (C6HnN4-NH-CHO), and methanyl-

Fig. 4.1. Gas chromatogram showing the sulfur- and nitrogen-species generated under simulated interstellar or circumstellar conditions, corresponding to a pho-tolyzed ice mixture of H2O:CH3OH:NH3:CO:CO2 = 2:1:1:1 after warm-up to room temperature. (Courtesy Munoz Caro et al., 2002).

aldehyd-HMT (C6HnN4-CHOH-CHO). These were the heaviest identified components of the residue. They also presented evidence for the formation of HMT at room temperature, and the important role of H2O ice as a catalyst for the formation of complex organic molecules.

Also the behavior of CO on an acidic ice, HCOOH, and of CO ices on a number of astrophysical grain analogs, including hydrogenated amorphous carbon (HAC), has been investigated. CO can be easily trapped in any hydrogen-bonding ice system, but the final desorption temperature of the CO depends intrinsically on the interplay between the crystallization and desorption behavior of the trapping matrix. High spectral resolution of the CO-ice band has been deconvolved into at least two and sometimes three components, consistent with new solid state features observed by Pontoppi-dan et al. (2003a) at 2175 cm"1.

Van Broekhuizen et al. (2004) explored laboratory simulations for the formation of OCN~ in interstellar ices. This ion, which can be probed through its vibrational stretching mode at 4.62 |m, has been observed toward a large number of mostly high-mass protostars. Several pathways were investigated involving either UV photolysis or thermal processing of relevant ice mixtures. Photolysis of CO with NH3 is too inefficient to account for the observed high abundances, but photolysis of CH3OH with NH3 showed an unexpectedly high

Fig. 4.2. Alanine enantiomers in the sample of Fig. 4.1. (Courtesy Munoz Caro et al., 2002).

OCN production rate. Thermal processing of mixtures involving HNCO can also meet the observational constraints.

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